download_queue: wake on completed downloads

[MikeMcQuaid]

• The parallel fetch loop paused a fixed 50ms per iteration, even when every download had already finished: Kernel#sleep was 27% of wall samples (~100ms) in brew prof --stackprof fetch for an already-cached formula, the largest single frame ahead of Kernel#require.
• Warm-cache downloads resolve in microseconds, so almost all of that sleeping was wasted on every brew fetch, brew install and brew upgrade, interactive or scripted, and even the final iteration slept after the last download had finished.
• Warm-cache brew fetch of a cached formula drops from 0.63-0.70s to 0.45-0.52s (~200ms, ~25-30%), measured both on a TTY and piped, and Kernel#sleep disappears from its profile entirely.
• Completed downloads are reported as they finish instead of on the next 50ms poll tick.
• Skip the end-of-iteration pause entirely when the partition left no downloads pending; previously even the final iteration slept.
• Wait on a Concurrent::Event signalled by an on_resolution! callback on every download future instead of sleeping, so warm caches never pay a poll interval.
• On animated TTYs the wait times out after 50ms, purely to keep redrawing the spinner and progress bars at a steady cadence while downloads are in flight; otherwise after 1s as a safety net.
• The event is reset before each wait and the remaining futures are re-checked in between, so a download resolving between the partition and the reset cannot cause a lost wakeup.

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Anthropic Fable 5 max with local review and testing.

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[Copilot]

Pull request overview

This PR improves Homebrew::DownloadQueue’s parallel fetch loop by replacing a fixed poll-and-sleep cadence with an event-driven wakeup when any download future resolves, eliminating wasted sleep time in warm-cache scenarios and reporting completions sooner.

Changes:

• Replace the fixed sleep 0.05 polling loop with a Concurrent::Event signaled by Future#on_resolution!, waking immediately on completion.
• Preserve spinner/progress redraw cadence on TTYs via a short wait timeout, while using a longer timeout for non-TTY output.
• Add an RSpec example asserting the queue no longer calls sleep in the parallel fetch path.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Library/Homebrew/download_queue.rb	Switches the parallel fetch loop from fixed-interval sleeping to an event-driven wait signaled by download future resolution.
Library/Homebrew/test/download_queue_spec.rb	Adds a spec ensuring the queue wakes on completion without calling sleep (non-TTY mode).

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